GB2199994A

GB2199994A - Rotary electrical machine having a flame proof electrically-insulated housing

Info

Publication number: GB2199994A
Application number: GB08700756A
Authority: GB
Inventors: Bernard Pomfret; Eric Dennis Darmon
Original assignee: Brush Electrical Machines Ltd
Current assignee: Brush Electrical Machines Ltd
Priority date: 1987-01-14
Filing date: 1987-01-14
Publication date: 1988-07-20
Anticipated expiration: 2007-01-14
Also published as: GB2199994B; GB8700756D0

Abstract

A rotary electrical machine having a rotor which is mounted in an electrically-insulated bearing 2 and a housing 3 forming a flame-proof enclosure around the electrically-insulated bearing 2 and having an end wall 4 in which there is an aperture through which a shaft 1 of the rotor extends outboard of the electrically-insulated bearing 2. The wall 4 may include a ring 8 or a bush having an inner annular surface closely surrounding the shaft 1 and within which the shaft 1 is freely rotatable. The diameter of the cylindrical surface and its axial length being such that together they define a restricted flame-proof path around the shaft in accordance with a regulation appertaining to the environment in which the machine is to be operated. There is also an electrically insulating barrier 9 between the shaft 1 and the ring 8 extending across the whole axial length of the cylindrical surface. Ring 8 may be replaced by a flanged plug (Fig 2). <IMAGE>

Description

ROTARY ELECTRICAL MACHINE HAVING AN ELECTRICALLY-INSULATED BEARING The invention relates to a rotary electrical machine having an electrically insulated bearing.

When a motor, or other rotary electrical machine is in operation a voltage is generated between the ends of the rotor shaft and thus where there is an electrical circuit through the shaft and bearings adjacent each end thereof, there is a risk that thk bearings could be damaged by pitting of the bearing surfaces as a result of sparking. It is therefore established practice to insulate electrically at least one of the end bearings, particularly in large machines where the voltage generated between the ends of the shaft is greater than in smaller-machines, thereby to interrupt an electrical circuit through the bearings and the shaft of the rotor.

Another reason for providing an insulated bearing is to reduce the occurrence of sparking where the machine is operating in an environment where an explosion could occur, for example, in a mine or a chemical or petrochemical works. Even where an insulated bearing is employed it might be possible for sparking to occur by bridging across the outside of the insulated bearing and so it is essential in machines which are intended for operation in such environments that the-insulated bearing should be positioned within a flame-proof enclosure.

Where the end of the shaft adjacent the insulated bearing does not extend through the outer casing of the machine, the outer casing may be closed by an end plate positioned outboard of said end of the shaft and secured to an adjacent part of the outer casing in a flame-proof manner, whereby the outer casing forms a flame-proof enclosure around -the insulated bearing.

Where the end of the shaft adjacent the insulated bearing passes through the adjacent end of the outer casing, the outer casing will not form a flame-proof enclosure around the insulated bearing unless a flame-proof seal is provided around the shaft end where the latter passes through the end of the outer casing. An object of the invention is to provide a flame-proof seal around the shaft end at a position outboard of an insulated bearing adjacent the shaft end.

According to the invention, a rotary electrical machine comprising a rotor mounted for rotation in a plurality of axially-spaced bearings, of which one is of an electrically-insulated kind, and a housing forming a flame-proof enclosure around the electricallyinsulated bearing and an end wall having an aperture therein through which a shaft of the rotor extends outboard of the electrically-insulated bearing, the wall defining the aperture having a cylindrical surface closely surrounding the shaft and within which the shaft is freely rotatable, the cylindrical surface having a diameter which is not greater than a maximum diameter and an axial length which is not less than a minimum axial length, said maximum diameter and said minimum axial length together defining a restricted flame-proof path around the shaft in accordance with a regulation appertaining to the environment in which the machine is to be operated, there also being an electrically insulating barrier between the shaft and the end wall of the housing extending across the whole axial length of the cylindrical surface.

The aperture in the end wall may be sealingly fitted with a bush which permits free rotation of the shaft therein, the bush having an internal diameter which is not greater than a maximum diameter and an axial length which is not less than a minimum axial length, said maximum diameter and said minimum axial length together defining a restricted flame-proof path around the shaft in accordance with a regulation appertaining to the environment in which the machine is to be operated, there also being an electrically insulating barrier between the shaft and the end wall of the housing extending across the whole axial length of the cylindrical surfaces.

Additionally there may be an electrically-insulating surface extending over the outer end face of the bush, the outer end surface of the end wall or the cylindrical surface of the shaft end outboard of the bush for a radial or axial distance respectively from the flame-proof path defined between the cylindrical surfaces of the bush or said end wall and the shaft end sufficient to prevent the establishment of a by-pass electrical path between the shaft and the end wall of the housing across the electrically-insulating barrier as a result of the presence of a contaminant material externally of the end wall.

By way of example, two alternative embodiments of an electrical machine in accordance with the invention are now described with reference to the accompanying drawings, in which: Figure 1 is an axial half-section through an outer end portion of the rotor and housing of the first embodiment, and Figure 2 is a similar view of the second embodiment.

Referring to Figure 1, the machine, e.g., an electric motor, has a rotor shaft 1- which is mounted in bearings of which only one is illustrated at 2. The bearing 2 is mounted in a housing 3. An annular end wall 4 is bolted to the housing 3 and forms an enclosure around the bearing 2. This bearing or bearing housing is of an electricallyinsulated type. For example it may be of a type in which the outer ring 5 is coated with a hard ceramic coating 6 or other insulating material extending along its outer cylindrical surface and end abutment faces. The ceramic coating 6 therefore prevents the passage of an electric current between the shaft 1 and the housing 3 and therefore prevents damage by pitting to the bearing surfaces of the bearing and the shaft.Although the insulated bearing 2 interrupts the passage of current between the shaft 1 and the housing 3 it may be possible for a spark to occur by bridging across the outside of the housing 3 or the end wall 4 and so where the motor is to be employed in an explosive or high fire-risk environment, e.g., in a mine or in a chemical or petro-chemical works, the enclosure of the bearing 2 by the end wall 4 should be flame-proof.

Where as in this example, the shaft 1 extends through an aperture in the end wall 4 a flame-proof seal has to be provided around the shaft. This presents a problem as the shaft 1 must pass through the aperture with sufficient clearance to enable the shaft 1 to turn freely. In this example, the projecting end of the shaft 1 carries a cooling fan rotor 7. However the invention is not restricted to an air-cooled machine as there may be another reason for the need for the shaft 1 to pass through the end wall 4.

The problem of how to make the end wall 4. flame-proof is solved in this embodiment by fitting a sealing bush into the aperture in the end wall 4. In this example the bush is a metal ring 8 which is an interference fit in the aperture and therefore forms a seal between the ring 8 and the end wall 4. The ring 8 is coated on its outer cylindrical surface over the whole axial length of the ring and on its end surfaces by a hard ceramic coating 9 or other insulating material, which may be similar to the coating 6 on the outer ring 5 of the bearing 2, providing an insulation layer which prevents sparking between the shaft 1 and the end wall 4. The ring 8 has an internal cylindrical wall which is a close-running co-axial fit around the shaft 2. The internal clearance of the ring 8 and its axial length are such that a restricted flame-proof path is provided between the ring and the shaft.There are regulations which control the radial thickness of-the path and its axial length according to the environment in which the machine is to be operated. Thus according to the environment, the internal diameter of the- ring 8 is not greater than the maximum and the axial length is not less than the minimum permitted for a flame-proof path permitted by the regulation appertaining to a particular environment. Thus by providing the electrically insulated ring 8 and the electrically insulated bearing 2 pitting of the shaft 1 or the bearing 2 and the- ring 8 is prevented and also the bearing 2 is contained within a specified flame-proof enclosure.

Another feature which is desirable is that the radial length of the layer of insulation on the outside face 10 of the ring 8 is sufficient to permit a deposit of a contaminant material without risk of the contaminant bridging between the shaft 1 and the metal of the end wall 4. This contaminant may be moisture which would quickly be dispersed by heat when the machine is in operation and therefore could not extend sufficiently far over the face 10 or it could be dust or dirt which would be cleaned from the face 10 before it has built up sufficiently to cause a by-pass of the insulation coating.

Figure 2 is similar to Figure 1 except that the ring 8 is replaced by a flanged plug 11 which is inserted into the aperture in the end wall 4 from the inside and therefore cannot be expelled in the event of an internal explosion. The plug 11 may be made of an insulating material and it may be fitted with a metal bush at its inner peripheral wall at 12. As in the first embodiment the internal diameter of the plug 11 and its axial length are made in accordance with the corresponding dimensions of the flame-proof path required by the regulations appertaining to the environment in which the machine is to be operated. Alternatively an insulating layer may be applied to the portion of the shaft 1 which is within the plug 11. As in the first example, an additional insulated surface is provided to allow for a deposit of contaminant.In this example, the additional insulation layer is provided on the shaft 13 and extends through a sufficient axial distance from the outside end of the plug 11.

Instead of fitting an aperture in the end wall of the housing through which a shaft passes with a bush defining together with the shaft the required flame-proof path, the aperture in the end wall may have a cylindrical surface which co-operates directly with the shaft to define the required flame-proof path and the electricallyinsulated barrier. In this case the barrier would be a coating of insulating material which extends through the axial 1 ength of the cylindrical surface. Also the coating may extend radially over at least the outer surface of the wall from the cylindrical surface around the whole circumferential length thereof for a sufficient radial distance to prevent the risk of a contaminant deposit bridging between the shaft and the end wall.

Provision is made in each example for re-lubrication of the bearing 2 without reducing the efficiency of the flame-proof enclosure.

A re-lubrication cylinder 14, feed tube 15 and lubrication nipple 16 are illustrated but are not described as they are the feature of our co-pending Application filed on the same day as this Application.

Claims

1. A rotary electrical machine comprising a rotor mounted for rotation in a plurality of axially-spaced bearings, of which one is of an electrically-insulated kind, and a housing forming a flame-proof enclosure around the electrically insulated bearing and an end wall having an aperture therein through which a shaft of the rotor extends outboard of the electrically-insulated bearing, the wall defining the aperture having a cylindrical surface closely surrounding the shaft and within which the shaft is freely rotatable, the cylindrical surface having a diameter which is not greater than a maximum diameter and an axial length which is not less than a minimum axial length, said maximum diameter and said minimum axial length together defining a restricted flame-proof path around the shaft in accordance with a regulation appertaining to the environment in which the machine is to be operated, there also being an electrically insulating barrier between the shaft and the end wall of the housing extending across the whole axial length of the cylindrical surface.

2. A machine as claimed in Claim 1 in which the aperture in the end wall is sealingly fitted with a bush which permits free rotation of the shaft therein, the bush having an internal diameter which is not greater than a maximum diameter and an axial length which is not less than a minimum axial length, said maximum diameter and said minimum axial length together defining a restricted flame-proof path around the shaft in accordance with a regulation appertaining to the environment in which the machine is to be operated, there also being an electrically insulating barrier between the shaft and the end wall of the housing extending across the whole axial length of the cylindrical surface.

3. A machine as claimed in Claim 2 having an electrically insulating surface extending over the outer end face of the bush, the outer end surface of the end wall or the cylindrical surface of the shaft end outboard of the bush for a radial or axial distance respectively from the flame-proof path defined between the cylindrical surfaces of the bush or said end wall and the shaft end sufficient to prevent the establishment of a by-pass electrical path between the shaft and the end wall of the housing across the electrically insulating barrier as a result of the presence of a contaminant material externally of the end wall.

4. A rotary electrical machine constructed and arranged substantially as described herein with reference to either Figure 1 or Figure 2 of the accompanying drawings.